The primary objective in the phosphate fertilizer industry is to convert the apatite in phosphate rock to a form in which the phosphorus is available to plants. Apatite is quite insoluble and, under most farming situations, is of little value as a supplier of nutrient phosphate. The most common method for making it available to plants is treatment with a mineral acid such as sulfuric, phosphoric, hydrochloric, or nitric. Use of nitric or hydrochloric acids, however, is of limited utility since these two acids yield mixtures of phosphoric acid and soluble calcium salts which are not easily separable from the phosphoric acid. The separation of the soluble calcium salt is important, since ammoniation of the unseparated mixtures to produce ammonium phosphate type fertilizers results in reversion of a major portion of the phosphate to an insoluble form not readily available as plant nutrients.
A method for manufacture of phosphoric acid from phosphate rock is disclosed in U.S. Pat. No. 4,108,957 to Michel. Crushed phosphate rock is mixed and digested with dilute phosphoric acid to form a slurry and convert the rock into monocalcium phosphate. Oxalic acid is thereafter added to precipitate the calcium as calcium oxalate. Phosphoric acid is recovered in a conventional manner and the calcium oxalate is treated with sulfuric acid to produce gypsum and recover oxalic acid.
Sulfuric acid as an acidulant results in the precipitation of the calcium component of the phosphate rock ores as an insoluble calcium sulfate salt, usually gypsum (CaSO.sub.4.2H.sub.2 O). This calcium sulfate salt is readily separable from the phosphoric acid, which acid can then be ammoniated to yeild soluble ammonium phosphate fertilizers. This practice has become an accepted method of producing nitrogen and phosphorus-containing fertilizers in the industry.
Although the use of sulfuric acid as a phosphate rock accidulant is one current practice, this alternative itself has several distinct disadvantages. A major disadvantage is the total dependence of the fertilizer industry upon low cost sulfur supplies for the production of sulfuric acid. However, recovery of sulfur from sulfur ore bodies by the well known Frasch process is energy intensive, and has resulted in an ever-increasing price for sulfur raw material as fuel prices continue to escalate. In addition, use of sulfuric acid results in the production of large tonnages of by-product gypsum. This by-product cannot presently be converted economically into a saleable product in the United States, and the disposal of this material represents a significant pollution control problem.
Therefore, a need exists for a process of phosphoric acid manufacture from phosphate rock, which process does not employ sulfuric acid or produce by-products which present significant pollution control problems. Further, a need exists for a method of manufacturing phosphoric acid from phosphate rock, which method can produce relatively pure phosphoric acid from low grade phosphate rock that includes large amounts of impurities and also produce usable by-products. Finally, a need exists for a process that can utilize basic starting materials such as air, synthesis gas and phosphate rock to produce phosphoric acid, ammonium phosphate and useful by-products.